Table of Contents

10.1 ADSL Connection Process

10.1.1 Bit Error Rate and Noise Margin
10.1.2 CAP RADSL Upstream/Downstream Payload Combinations
10.1.3 Training Procedure

10.2 Understanding ATU-C Train Parameters

10.2.1 Receive Signal Quality
10.2.2 Receiver Gain
10.2.3 Transmit Power

10.3 Understanding Performance Counters
10.4 Performance Management
10.5 Current Connection Activity

ATU-C Port Properties Status Tab
Active Connections Window

System Operation and Performance (Direct Connect)

This chapter covers basic system operation and the performance metrics captured as a result of system operation in a Direct Connect configuration. Understanding performance statistics becomes easier if you understand the ADSL connection process. This process is explained directly below.

10.1 ADSL Connection Process

The Carrierless AM/PM (CAP) rate-adaptive ADSL (RADSL) transceivers used in Cisco's ATU-Cs and ATU-Rs are capable of training at a number of different discrete settings within each of four baud rates. Because the upstream and downstream data paths are transmitted in different frequencies, they are trained independently.

Cisco's CAP RADSL implementation supports a number of ADSL training options that can be used to control subscriber traffic. The following background information provides an overview of the training process.

10.1.1 Bit Error Rate and Noise Margin

In general, ADSL transceivers seek an upstream and downstream data rate that can be maintained as long as the amount of line noise does not cause a bit error rate (BER) in excess of 1x10^-7. Line noise is a function of reach and disturbers. As reach and/or noise in the loop increases, upstream and downstream payload rates decrease. Also, the thinner the copper wire, the more susceptible it is to noise. As line noise is not constant, a certain amount of line noise fluctuation is tolerated without affecting the trained rates. This fluctuation is known as noise margin. Noise margin must be supplied to the transceivers when using the multi-band method of training (multi-band is discussed further below).

Because upstream and downstream payloads are transmitted in different frequency ranges, the two payload rates are established independently. Layer 2 protocol data units (PDUs) are encoded into Layer 1 (ADSL) transmission frequencies using baud rates (also known as symbol rates) and constellations. Different baud rates are used to achieve different payload rates. CAP RADSL transceivers support one baud rate for establishing upstream payload rate - 136 Kbaud, and three baud rates for establishing downstream payload rate: 340, 680, and 952 Kbaud. These baud rates enable CAP RADSL implementations to support downstream payload rates ranging from 7168 Kbps to 640 Kbps, and upstream data rates ranging from 1088 Kbps to 91 Kbps.

Because the user can change the noise margins for a subscriber line, a line which was training before may not train at the new margin. Therefore, a warning is issued to alert the user to use caution when changing the margin.

Cisco recommends that the margin be set for 6 dB upstream and 3 dB downstream to provide performance similar to the pre 2.2 Cisco 6100 releases. The default values for margins in ViewRunner for HP OpenView are 6dB upstream and 3dB downstream.

10.1.2 CAP RADSL Upstream/Downstream Payload Combinations

Valid CAP RADSL constellations are 256UC, 256, 128, 64, 32, 16, 8, and 8ER. Note that the 128, 32, 8, and 8 ER constellations are not supported with downstream baud rates of 952 or 680. Because of this, some upstream/downstream payload combinations cannot be achieved. The following table shows valid upstream and downstream payload combinations used in the initial releases of
Cisco's 6100 and 675 products:

10.1.3 Training Procedure

The following rules apply to the training sequence:

• The receive side of each data stream controls the train rate of that stream. The ATU-C controls the upstream data rate selection, and the ATU-R controls the downstream data rate selection.

• The above rule can be circumvented by setting the ATU's data rate for a give stream to the maximum data rate mode. When one end is set to the max, the other end by default selects the rate.

• ATU-Rs are preset to:

Upstream data rate = Max

Downstream data rate = Max

This means that control for the downstream data rate is relinquished to the ATU-C (the ATU-C controls the upstream data rate setting by default).

With the above rules in place, the ATU-C has complete control over upstream and downstream data rate selection. Valid upstream and downstream data rate combinations are selected from within the ViewRunner for HP OpenView management interface and communicated to the ATU-C per the following procedure:

• A select option provides a list box dialog with valid upstream/downstream payload combinations (of which there are currently 80 per the above table). Once you select an option, the data rate combination populates the maximum allowed upstream and downstream rates in the previous dialog.

• Once rates are selected and applied, these values are written into the MIB. At connection request time, the Cisco 6100 connection manager queries the MIB and provides programmed data rates as ceilings to the ATU-C.

• If no ceilings are provided, the ATU-C attempts to train at the minimum upstream and downstream data rates, 91 kbps and 640 kbps, respectively.

• If ceilings are provided, the ATU-C uses the multi-band terminal algorithm to determine the best upstream and downstream data rate combination equal to or just below the operator-defined ceilings.

• With Reliable Dial, if ceilings are provided, the second and following attempts to train will remember the information stored during the first train. Then the new train will take only seconds since the multi-band terminal algorithm will not have to be run again.

In a subsequent product release, Cisco's QuickDial will be supported. With QuickDial, this train sequence only occurs once per subscriber (barring unusual changes in loop characteristics). After the initial train sequence, training coefficients are stored in the SC, enabling subsequent trains to occur rapidly.

10.2 Understanding ATU-C Train Parameters

This section includes information related to receive signal quality, receiver gain, and transmit power where it applies to noise margins on the Cisco 6100s. Each of these trained line attributes is displayed in ViewRunner in the ATU-C Module Properties > Configuration dialog.

The Cisco 6100 operator can now change the noise margin (upstream and downstream) for each subscriber line. The following window shows the ATU-C Module Properties window for a session in progress. The various train parameters are identified here.

10.2.1 Receive Signal Quality

Receive signal quality (SQ) is a measure of the signal quality of the upstream channel (from ATU-R, in the case, the Cisco 675). It is represented in units of dB.

This value is used to estimate the bit error rate (BER) or signal-to-noise ratio (SNR) margin of the received data. It takes into account the total signal-to-interference ratio (SIR), where the interference includes background noise, cross-talk, residual inter-symbol interference, residual echo from the neighboring upstream or downstream, and distortion.

For 7 Kft loops, it is fairly common to observe receive SQ of 37-44 dB, with average interference and a 6 dB noise margin setting. For 10 Kft loops, an SQ range of 32-35 dB would be common. As loop length increases, the SQ will decrease. Long loops (12-15 Kft) or loops that have bridge taps could have an SQ in the dB range of the low 30s. Very long loops (+ 15 Kft) can even train with an SQ as low as 20 dB @ dB margin. If noise margin is reduced to zero, even longer loops could train with an SQ of as low as 12 dB.

The above data is provided as a perspective on likely observed SQs. Stating explicit valid SQ ranges for a given loop reach is, however, not particularly valuable. The number and variety of interferers, wiring (outside plant and in-home), etc. creates so many different scenarios that specific data is not always helpful if trying to institute, for example, a corrective action by moving a pair to a different binder group as a remedy.

Note also that one of the great values of RADSL is that it removes the need for the operator to figure out these values for optimum performance. So, they are presented to primarily as an indicator that the trained loop is exhibiting expected characteristics, rather than necessarily for troubleshooting purposes. For a given training session, it should be remembered that if RADSL cannot overcome loop characteristics such that even the lowest upstream/downstream data rates are not supportable, these layer 1 attributes will not be displayed anyway.

It is also important to understand the SQ is not a function of the data rates. In fact the opposite is true. Data rates are a function of the SQ. For a given requested upstream/downstream data rate combination (as selected in ViewRunner), if the transceivers cannot maintain the data rates to a 10^-7 BER (with a 6 dB noise margin insertion), then the transceivers will seek the next lowest data rate combination where the BER can be preserved.

ViewRunner for HP OpenView allows the operator to select a noise margin. This is valuable if, for example, a subscriber requests 7.168 Mbps x 1.088 Mbps, but the loop quality is not sufficient to hold the BER to less than or equal to 10^-7, the subscriber's RADSL train could perhaps settle to
6.272 Mbps x 1.088 Mbps. In this scenario it could be that interference is causing the downstream channel to have just enough noise to where the 7.168 Mbps data rate cannot quite be achieved with the 6 dB margin hit. Reducing the noise margin (that the transceivers must take into account for determining the best data rates to lock into) to 3 dB might, however enable the full 7 x 1 service to be provided.

10.2.2 Receiver Gain

Receiver gain is a measure of loop attenuation over the entire CAP RADSL frequency spectrum. The ATU-C has an algorithm that enables it to boost receiver gain such that attenuation can be corrected for proper support of a given receive data rate.

The ATU-C algorithm attempts to keep gain to a minimum to prevent Near-End Cross-Talk (NEXT). However, if loop conditions warrant, the algorithm must boost gain enough to insure the minimum signal level required is received.

Factors that tend to cause receiver gain to be boosted are associated with loop impairment, specifically attenuation (length) and/or the presence of bridge taps. Receiver gain is fairly independent of noise. So if the gain is high, it may mean indicate a long loop, or a loop with a significant bridge tap.

Bridge taps do not affect the upstream channel as much as the downstream channel. Since we only display upstream receiver gain in ViewRunner, a higher than normal gain is most likely an indicator of loop length.

The following receive gains have been observed in testing:

Table 10-3: Reach and Receiver Gain

Reach (Kft)	Receiver Gain (dB)
16	42
14.5	39
11	27
9	21
8.7	19

10.2.3 Transmit Power

Transmit power is a measure of the downstream Power Spectral Density (PSD) mask. T1E1/97-104R2a states that the PSD for the downstream channel shall have an upper limit of
-40 dBm/Hz in the nominal passband region with no variation exceeding -37 dBm/Hz. However, the maximum transmit power is really the decision of each network provider. As power is boosted, reach can be extended for a given data rate. At the same time, however, the boosted signal can become a greater disturber for other services.

T1E1/97-104R2a also specifies that the PSD for the upstream channel shall have an upper limit of -38 dBm/Hz nominal with no variation exceeding -35 dBm/Hz.

The ATU-R's transceiver determines its transmit power during the training process as it tries to fulfill the upstream data rate request. It tends to try to support the requested data rate at the lowest possible transmit power so that near cross talk in the Cisco 6100 is minimized

10.3 Understanding Performance Counters

The Cisco 6100 is equipped with train counters used to capture system performance statistics in a Direct Connect configuration.

After the connection has been established, the training sequence as described above begins.

If the ATU-C modem and ATU-R modem train, a successful train counter increments for the ATU-C modem. If the ATU-C modem and ATU-R modem fail to train, a failed train counter increments for the ATU-C modem. Train counter statistics can be viewed via the Performance Management window.

10.4 Performance Management

To check the performance of the ATU-Cs in a Direct Connect configuration, right click on the multiplexer chassis (MC) and select the 6100 Performance option. The Performance Management window appears:

Table 10-4: Performance Management Field Descriptions

Field	Description
ATU-C Port	Displays the chassis, slot and port number of the ATU-C unit.
Successful Trains	Displays the number of times the associated ATU-C module has trained successfully.
Failed Trains	Displays the number of times the associated ATU-C module has failed to train successfully.
Training Statistics	Allows you to display ATU-C training data. This option is disabled unless a line within the statistics table is selected.

If an ATU-C module fails to train, you may have provisioned it incorrectly. See "Service Provisioning (Direct Connect)" for more information on provisioning.

The existing table that shows performance statistics, such as successful trains, failed trains, etc., has been modified to allow single lines of the table to be selected. Once you have selected a line of the table for an ATU-C module, a button on the performance dialog enables to allow you to display a graphical representation of the Bit Allocation, SNR, and Receiver Gain for that module. The graph can be refreshed or closed using available buttons on the display. The range of the training statistics display is the same as that specified in the MIB.

In ViewRunner for HP OpenView, the user can open as many ATU-C graphs as are allowed by the windows system limit. The next three figures show each of the types of statistics displayed for DMT-1.

10.5 Current Connection Activity

Two current connection activity displays are provided:

• ATU-C port properties status tab

• Active Connections View

ATU-C Port Properties Status Tab

For ATU-Cs in a Direct Connect configuration, the port status window displays state information.

Active Connections Window

The Active Connections window displays a snapshot of all currently active connections in the
Cisco 6100. To open the active connections dialog, right click on the MC and select the Active Connections option. The following window appears. The next three figures present the Active Connections window first in the far left position, then scrolling right for the last two figures.

You can use the logical service oriented navigation to go directly to the desired entity by clicking on any hyperlink in the screen. describes the fields on the screen.

Table 10-5: Active Connections Field Descriptions

Field	Description
Subscriber ID	Displays the identifier by which the subscriber is known.
ATU-C Port	Displays the chassis, slot and port number to which the line port is connected.
Modem State	Displays whether the modem is trained or training.
Actual Up	Displays the actual upstream train rate at which the subscriber is training. This rate may never be higher than the Provisioned Up rate.
Actual Down	Displays the actual downstream train rate at which the subscriber is training. This rate may never be higher than the Provisioned Down rate.
Rx SNR	Signal to noise ratio for ADSL upstream (receive side) data channel.
Provisioned Up	Displays the upstream rate set by the CO, the maximum upstream rate at which the subscriber can train.
Provisioned Down	Displays the downstream rate set by the CO, the maximum downstream rate at which the subscriber can train.
Provisioned Up Margin	Displays the upstream noise margin provisioned by the operator.
Provisioned Down Margin	Displays the downstream noise margin provisioned by the operator.
Actual Up Margin	Displays the upstream noise margin actually occurring within that provisioned by the operator.
Actual Down Margin	Displays the downstream noise margin actually occurring within that provisioned by the operator.

Posted: Fri Oct 8 13:08:09 PDT 1999
Copyright 1989-1999©Cisco Systems Inc.